Austenitic stainless steel



Patented Dec. 7, 1948 UNITED v TES RATEN T OFF I CE AUSTENITIC. STAINLESS STEEL "Joye C. 'Haun, San Francisco,-Galif., and Randolph Simpson, Portland, reg., assignors to .Electric SteeLFoundry, Portland,-Oreg., a corporation of Oregon No= Drawing. Application April '26, 1945, Serial No. 590,510

2Claims.

This invention is concerned'with improving the corrosion resistance-of austenitic stainless steels having significant contents of molybdenum, and

-..nificant contents of. molybdenum.

Generally speaking, the higher the molybdenum contentof a .givensteelthe more may be .itsboroncontent to the enhancement of its. corrosion resistance .without concomitant undesiriable modification of .its requisite physicals. .Oftentimes one of. the. physicals-.machinability-- is definitely improved. bythe same small addition of boronthatlie-responsible for a desirable increaseinthe corrosion resistance of such a steel.

Thus far our invention has evidenced great industrial value inthe improvement of austenitic stainless steels containing molybdenum, e. g.,

those steels containing, essenti'ally,.'from 17 toZO per cent chromium, 7 to 14 per cent nickel, and 1.5 to per centmol-ybdenum; but various and sundry'other austenitic stainless steels contain- .in molybdenum are improved asto corrosion resistance, without objectionable .mOdificatiQn of ,their characteristic physicals, by additions i boron in very small. percentages determinable "by procedures like or akin tothose hereinafter discussed.

Anvaustenitic stainlessc=steel= of the class men tfionedinthe lastnpreceding paragraph will have its corrosion resistance substantially increased by the addition thereto of boron constituting from about .001 per cent to about .100 per cent of the resultant alloy. Austenitic stainless steels containing molybdenum, other than stainless steels of the 18-8 molybdenum type, likewise will have their abilities to resist corrosion substantially increased by easily determinable boron additions ranging from about .001 per cent to about .500 per cent of the resultant alloys. We contemplate that the molybdenum contents of steels of the present invention will be as low as about 1 per cent.

We shall now record our procedure and results .in determining what'boron additions should be made tocertain representativev steels containing molybdenum in order substantially to increase their resistance to the corrosive action or certain reducing acids.

A heatof a steel, comprising substantially 19.0 per cent chromium, 9.0 per cent nickel, and 3.0 per cent molybdenum, wastaken from an induction furnace, andutilized in the pouring of test bars numbered and having boron contents as follows:

No. No. No. No. No. No. No. No. No.

The boron containing test bars conveniently were givenrtheir respective boron contents by adding NOS-contained no boron 1'T:G3-A-contained .002% boron '1703-Bcontained 004% boron l'7=03--(-J--contained 006% boron 1'703-Dcontained 008% boron -1703-'Eoontained .O10% boron =1703 -Fcontained 030% boron 1703-Gcontained .050 boron '1703- -Hcontained .100%' boron ssweighted quantities of ferro-boron to different =portionszof the heat.

Numerous tests in 33 per cent sulphuric acid, and in: particularly corrosive paper mill solutions, were run "upon samples taken'from such test bars *todetermine corrosion resistance. The results of'these testsestablished that alloy No. 1703, containing no boron, was much less corrosion resistant-thanany of the samples containing boron; the'preferredrange of boron from the standpoint of-over all corrosion resistance, being from .004 to .0l0'per-' cent. Alloy 1703-A, which contained the smallest addition of boron, showed much better corrosion resistance than alloy 1703. Alloys Nos. 1703-F, 1703-G and 1703-1-1, containing more boron than alloy No. 1703-13, showed better overall corrosion resistance than did alloy No. 1703 but did not show as good resistance as alloys Nos. 1703-B, 1703-0, 1703-D, and 1703-13. In respect to corrosion resistance, alloys Nos. 1703-B, 1703-0 and 1703-D definitely were superior to the others.

The sulphuric acid tests of the last preceding paragraph were repeated after reducing materials such as ferrous sulphate, other metal sulphates, and small amounts of chloride salts, had been added to the 33 per cent sulphuric acid. Again 3 it was found that alloy Nos. 1703-3, 1703-C, 1'703-D and 1703-E demonstrated much better corrosion resistance than did any of the others.

Much of the corrosion resistance of the socalled 18-8 molybdenum steels is believed to be attributable to the heat treatment they customarily receive before being put into servicesuch heat treatment serving to trap carbides in the matrix and thus prevent them from migrating to the austenitic grain boundaries with consequent reduction of the chromium content of the matrix. With this in mind, samples from test bars Nos. 1703, 1'703-A, 1703-D, and 1703-11, after receiving the customary heat treatment, were sensitized by heating in a, furnace to 1200 F. and were held at that temperature for a considerable time and then allowed to cool in the furnace. This sensitizing treatment neutralized the efiects of the customary heat treatment and caused carbides to precipitatethese carbides predominating at the grain boundaries of the delta phase ferrite pools. (The same effects would be obtained in welding without subsequent heat treatment.) The samples were next cleaned and polished. They were then tested in 33%% sulphuric acid; and, after fourteen days immersion, it was found that alloy No. 1703 lost approximately 11 per cent in weight, whereas alloys Nos. 1703-A, 1703-D and 1'703-H lost no weight whatever. The surface exposed on all samples tested was identical. These results very clearly indicate that in so far as the 18-8 molybdenum steels are concerned, boron contributes greatly to their corrosion resistance even after the carbides have been precipitated.

Many tests made by us'have shown that the addition of boron to 18-8 molybdenum steel does not undesirably afiect physicals until the boron content exceeds .05%. steel, as the term is used herein, we mean steels having a chromium content ranging from 16 to 20 per cent, a nickel content ranging from '7 to 14 per cent, and a molybdenum content ranging from 1.5 to 5.0 per cent.

It has been found, in the use of fabricated stainless steels, that the corrosion resistance to acids under reducing conditions of those austenitic stainless steels having molybdenum con tents of 2.5 per cent or higher is much superior to those having molybdenum contents below 2.5%. The corrosion resistance of these austenitic stainless steels increases materially the higher the molybdenum content is carried up toward 3.5 per cent. However, such steels, when.

they contain more than 2.5 per cent of molybdenum, are very difiicult to roll due to their tendency to tear. Therefore, tests were made to determine how small additions of boron would affect an 18-8 molybdenum steel containing approximately 1.6 per cent of molybdenum. Sam- By 18-8 molybdenum 4 ples of such steel bore numbers and were characterized by boron contents as follows:

No. K-227 -C0ntained no boron No. K-227-A-Contained .002% boron No. K-227-B-.Contained .004% boron No. K-227-CContained .006% boron No. K-227-D-Contained 008% boron No. K-227E-Contained 010% boron Per cent No. K-22'7-B (containing 004% boron) 8 No. K-227-C (containing .006% boron) 15 No. K-22'7-D (containing 008% boron) 22 No. K-227-E (containing .010% boron) approximately 25 Having thus disclosed our invention and how it may be practicably and profitably utilized, what we claim as new and desireto secure by Letters Patent of the United States is:

1. An austenitic stainless steel, characterize by great resistance to corrosion under reducin conditions and additionally characterized by its rollability, including a chromium content of from 16% to 20%, a nickel content of from 7% to 14%, a molybdenum content of from 1.5% to 2.5%, a boron content of from .001% to .006%, and the balance iron and impurities.

2. An austenitic stainless steel, characterized by great resistance to corrosion under reducing conditions, and additionally characterized by its rollability, including a chromium content of from 16% to 20%, a nickel content of from 7% to 14%, a molybdenum content of approximately 1.6%, a boron content of approximately .002%, and the balance iron and impurities.

JOYE C. HAUN. RANDOLPH SIMPSON.

REFERENCES CITED The following references are of record in the 

